This invention relates in general to video reproduction apparatus which processes a high resolution time-division-multiplex (TDM) video signal. More particularly, this invention relates to the recording of a field of a high resolution TDM video signal in two adjacent tracks on magnetic tape. A head switch signal is included in the horizontal sync service signal of a horizontal line signal recorded at the end of the first track to effect switching between video signals played back from the first and second tracks in order to minimize skew error and image distortion and degradation problems.
Purchases of video reproduction apparatus and, in particular, video cassette recorders (VCR) have increased dramatically in recent years. All of the popular consumer VCR formats (VHS, Beta, 8 mm) use helical scan recording in order to obtain high writing speeds so that high frequency video signals may be recorded on tape, and in order to attain long recording times (for example, up to 8 hours in the VHS format). Conventionally, a rotating head drum carries a pair of magnetic heads which record a video signal on slant tracks on magnetic tape which is moved past the rapidly rotating magnetic heads. Each head records a field of video signals on a single track. Thus, the two fields of a frame are recorded in two adjacent slant tracks on the tape. In order to reduce cost and complexity of the VCR, the composite video signal is reformatted to separate the chrominance and luminance signals in a frequency fashion, in order to reduce time base errors which result in reproduced image degradation. In addition, the bandwidths of the luminance and chrominance signals are usually reduced to reduce cost and equipment complexity.
In the recording mode, the video signal is conventionally applied to both magnetic heads for a brief overlap period so that the video signal recorded at the end of one track by one magnetic head is also recorded at the start of an adjacent track by the other magnetic head. When the tape is played back during this overlap, the magnetic heads produce two video signals which are fed into two parallel transmission paths only one of which is connected to an output path. A switching signal to connect one or the other of the transmission paths to the output path is developed by magnetic pulse generator coupled to the rotating head drum. Switching between video signals is chosen to fall in the center of the overlap period so that the combined output signal has a uniform envelope, with no interruptions. The head switching is effected near the bottom of the field or in the vertical blanking period. Because of differences in tension on the magnetic tape between the time of recording and the time of playback (i.e. skew error), a loss of horizontal synchronization may occur which results in image distortion and jitter.
Head switching becomes especially critical when a field of a high resolution video signal is recorded on two adjacent tracks on magnetic tape so that switching occurs in the middle of the field. Such a high resolution video reproduction system has been proposed in commonly-assigned, copending U.S. patent application Ser. No. 725,873, by Carl Schauffele, entitled VIDEO REPRODUCTION APPARATUS, filed Apr. 22, 1985. As disclosed in this patent application, a bi-modal video cassette recorder is selectively operable in a conventional mode at reduced signal bandwidth and in a high resolution time-division multiplex mode at increased signal bandwidth. In the high resolution mode, a field of video signals is recorded on tape which is moved twice as fast as in the conventional format recording, and by means of a magnetic transducer which is rotated at twice the rotational speed as the conventional mode. Thus, in the TDM format, the upper half of a video field is recorded on one track, and the lower of a video field is recorded on an adjacent track. Since head switching between the upper half field signal and the lower half field signal occurs in the middle of the field during playback, any skew errors that result will cause image degradation in the middle of the viewing area of the reproduced image, and will be especially annoying to a viewer who is expecting a high quality, high resolution image.